Dry wall or gypsum board is commonly used in the erection of interior walls in commercial, residential and other building structures. Dry wall is effective when used to enclose rooms subject to normal humidity and surface moisture conditions that occur in many of the rooms commonly found in offices, shops, residences and other buildings. However, bathrooms, basements and certain areas of residential and commercial kitchens may pose potential moisture problems for interior walls constructed from dry wall wallboard.
When dry wall is used in bathrooms, for example, tile may be adhered directly to the dry wall. Alternatively, some bathrooms utilize prefabricated modular stalls and/or bathtubs which may be adhered to the dry wall in the bathing areas of the room. Because hot baths and, especially, hot showers produce steam, bathrooms are frequently exposed to periods of very high humidity. Additionally, the basins and bathing areas thereof are susceptible to localized collection of surface moisture in the form of small pools or puddles of water. If cracks are present in the tile grout or if the seams between the dry wall and the basins or prefabricated bathing area components are not completely sealed, the steam or puddled surface water may come into contact with the dry wall.
The opposed faces of dry wall are typically covered with paper which is suitable for receiving paints, primers and tile adhesives. However, such paper also has a tendency to absorb water. As the water is absorbed by the paper, it comes into contact with the gypsum core of the dry wall. Gypsum is a hygroscopic material. The gypsum core therefore absorbs moisture that passes through the facing paper. Over the course of time the level of water absorption may degrade the structural integrity of the dry wall board. If the water damage becomes excessive, some or all of the board may require replacement, which may be an especially laborious task in the bathing areas of a bathroom.
Because they are effectively immune to water damage, cementitious boards have been employed as alternatives to dry wall in particularly humid and wet rooms. Cementitious boards may be fabricated to assume essentially the same dimensions and weight, as well as support the same sorts of facing materials, as conventional dry wall. As is known, concrete and similar cementitious materials have far greater compressive strength than tensile strength. This phenomenon mandates that cementitious boards and similar, relatively thin, panel-like cementitious objects be handled with care during transport. More specifically, unless sufficiently reinforced such boards must be carried substantially vertically, i.e., with their opposed faces extending substantially perpendicularly to the ground or floor surface. If carried substantially horizontally, i.e., with the supposed board faces extending substantially parallel to the ground surface such as would occur if opposite end edges or opposite side edges of the board are supported by two or more workers, the material in the upper regions of the board (in the thickness dimensions of the board) would be in compression and the material in the lower regions of the board would be in tension. If the tensile forces exceed the tensile strength of the cementitious material, the board may snap during transport. Alternatively, although less overtly catastrophic, radiant cracking may occur in the lower regions of the board which may preclude its installation or, if installed, might greatly comprise its bearing capacity and service life. Moreover, reinforcement should be of sufficient durability that it continues to strengthen and toughen the board over the typical projected 20-40 year service life of the board.
Various means have been proposed for reinforcing cementitious boards. Typically, the reinforcement comprises an open grid structure whose central plane is embedded approximately 1/32 to 1/16 inch beneath each face of the ordinarily ½ to ⅝ inch thick board. For example, open mesh woven polypropylene has been used for this purpose because of its resistance to water and the alkaline chemistries of Portland cement concrete and similar cementitious materials. However, because of the comparatively low modulus of elasticity of polyolefins such as polypropylene and polyethylene, which is on the order of about 10,000 to about 75,000 psi, such materials experience high strain under the tensile loads which can occur due to improper handling of the cementitious board. As tensile reinforcement, therefore, polypropylene grids are of limited practical use.
High modulus of elasticity materials have been proposed as reinforcement for an assortment of products and structures. U.S. Pat. Nos. 4,537,610; 5,011,523; 5,316,561; 5,328,493; 5,425,796; 5,451,355 and 5,454,846, for example, describe various means by which glass fiber rovings or yarns are coated with thermoplastic organic materials to produce composite strands or yarns which can be incorporated into end products. Among these patents, U.S. Pat. No. 5,011,523 proposes the possibility of using such composite rovings or yarns to make fabrics. The fabrics are indicated as being transformable into rigid plates following heat treatment and subsequent cooling of the fabrics. There is no discussion in these documents, however, of whether and how such composite strands or the products made therefrom might be used to reinforce building panels such as cementitious boards.
U.S. Pat. Nos. 4,699,542; 4,957,390; 5,110,627; 5,246,306 and 5,393,559 disclose semi-rigid open mesh grid fabrics comprised of fiberglass warp and weft rovings impregnated with resinous materials. The fabrics are stated as being designed for use in reinforcing asphaltic concrete over layers in roadway constructions. The fiberglass rovings are disclosed as having weights of from about 300 to about 5,000 tex, although ECR or E glass rovings of 2,200 tex are disclosed as the preferred material in each of these patents. Indeed, the commercial embodiment of the products disclosed in these patents, which is produced under the trademark GLASGRID® by Bay Mills Limited of St. Catherines, Ontario, Canada, employs glass rovings of 2000 tex. Rovings of this considerable weight, when impregnated with protective resin, produce a fabric whose strand thickness is approximately 0.04 inch. At the intersection of the warp and weft strands, the fabric thickness rises to approximately 0.056 inch. Fabrics of such substantial thickness may be used without concern in environments such as roadway reinforcement wherein the fabric is to be embedded beneath or between roadway layers of greater than one inch or, more typically, at least about two inches or more in thickness. Indeed, in such installations, fabrics of considerable thickness and grid opening size are desirable. They permit substantial contact between the underlying and overlaid roadway layers which is necessary for effective transfer of potentially damaging stresses from the pavement to the high modulus glass fibers in the fabric.
As noted hereinabove, the reinforcement provided in existing cementitious boards is typically embedded approximately 1/32 to 1/16 inch beneath each face of the boards. By locating the reinforcement so close to the surface of the board faces, the tensile stress transfer from the concrete to the reinforcement is optimized. It is the concrete material which is closest the board faces, i.e., the “skin” concrete, that is potentially subject to the greatest tensile forces and, therefore, in greatest need of tensile stress relief due to improper handling of the board. If reinforcement is embedded much more deeply than about 1/16 inch beneath the board faces, the reinforcement becomes situated too close to the neutral axis of the board. Hence, the flexural modulus of the board is thus reduced, and the board becomes more flimsy and prone to surface cracking. By maintaining the reinforcement close to the board faces, the board is stiffened in much the same way that the parallel flanges strengthen an I-beam.
The rather thick GLASGRID® fabrics thus cannot be used for reinforcement of relatively thin, prefabricated cementitious boards designed for use as interior building wall panels. Firstly, their comparatively thick strands or rovings present a pronounced undulating substrate with which the thin outer “skin” concrete layer of the board must interengage and adhere. The peaks and valleys produced by the thick GLASGRID® fabrics, especially at the warp and weft strand intersections, would produce localized areas where the skin concrete would be less than about 1/16 inch in thickness, which is believed to be the minimum thickness to prevent spalling of the skin concrete. Stated differently, the GLASGRID® fabrics would create a low shear plane between the skin and core concrete of the cementitious boards. The low shear plane, in turn, would hinder retention of the skin concrete (and any facing materials borne thereby such as tile or the like) to the core concrete.
Secondly, if embedded more deeply within the board, the ability of the GLASGRID® fabrics to provide effective tensile stress relief in the skin concrete, would be compromised if not negated.
Bay Mills Limited also produces an open grid fiberglass yarns or fabric coated with polyvinyl chloride (PVC) for use as cementitious board reinforcement. This grid uses fiberglass rovings which are of a size and weight suitable for embedment at an appropriate depth between the outer skin concrete and core concrete of cementitious boards. Portland cement concrete and similar concretes are grainy, alkaline compositions which can damage unprotected glass fibers. PVC is highly alkali and water resistant and, therefore, seemingly suitable for embedment in Portland cement and similar concrete matrix material to protect the fiberglass rovings of the fabric from alkali and water damage, as well as etching from the aggregate particles in the concrete matrix. Because it is a rigid material, PVC requires plasticizers such as ester oils to be rendered sufficiently fluid to be applied and function as a coating. It has been observed, however, that ester oils and related plasticizers are themselves susceptible to alkali attack. The concrete matrix material thus tends to gradually degrade the ester oil modified PVC coating causing small openings to be formed in the coating. Alkali then enters these openings and causes degradation of the glass fiber rovings. The process may proceed to an extent where deterioration of the rovings prevents installation of a cementitious board or, if already installed, replacement of the board.
U.S. Pat. No. 5,552,207 describes wall facing comprising an open grid, resin impregnated, fiberglass fabric which is affixed to a rigid foam insulation board and covered by and embedded within stucco or stucco-like material. The wall facing may be prepared either in situ on the outside of a building or in the form of prefabricated panels which may be bonded to a building wall. The wall facing, including the prefabricated panel embodiments thereof, is attached to a pre-existing wall and is not itself used as a wall panel in the manner, for example, of dry wall or the cementitious boards of the present invention. Indeed, the wall facing may be affixed to dry wall or cementitious boards but cannot be used in lieu thereof because of the low bending strength of its plastic foam backing board. A wall constructed solely of such facing would likely be destroyed as a result of minor impacts thereagainst, including the sorts of impacts routinely absorbed by dry wall and cementitious wall panel boards.
A need exists, therefore, for high modulus reinforcement for cementitious boards and similar construction panels which is of suitable weight and thickness to be embedded near the outer faces of such panels and which is highly resistant to alkali and water attack.
Further advantages exist for reliable methods for manufacturing such reinforcement, boards incorporating the reinforcement and methods for manufacturing such boards.